Replication kinetics and shedding of very virulent Marek's disease virus and vaccinal Rispens/CVI988 virus during single and mixed infections varying in order and interval between infections

•We investigated the effects of co-infections at various vaccination challenge intervals (VCI) between Rispens/CVI988 and vvMDV.•Rispens significantly reduced the viral load of vvMDV in PBL and feather cells and shedding in dust.•vvMDV significantly reduced the viral load of Rispens in PBL and feather cells but not in dust.•VCI significantly influenced these relationships.•Differences between two viruses and their effects on each other were greatest in PBL and feathers, and least in dust.

Vaccination is thought to contribute to an evolution in virulence of the Marek's disease virus (MDV) as vaccines prevent disease but not infection. We investigated the effects of co-infections at various intervals between Rispens/CVI988 vaccine virus (Rispens) and very virulent MDV (vvMDV) on the replication and shedding of each virus. The experiment used 600 ISA Brown layer chickens in 24 isolators with all treatments replicated in two isolators. Chickens were vaccinated with Rispens and/or challenged with the vvMDV isolate 02LAR on days 0, 5, or 10 post hatching providing vaccination to challenge intervals (VCI) of −10, −5, 0, 5 or 10 days with the negative values indicating challenge prior to vaccination. Peripheral blood lymphocytes (PBL), feathers and isolator exhaust dust were sampled between 7 and 56 days post infection (dpi) and subjected to quantitative real-time polymerase chain reaction (qPCR) to differentiate the two viruses. Overall Rispens significantly reduced the viral load of vvMDV in PBL and feather cells and shedding in dust. Similarly vvMDV significantly reduced the viral load of Rispens in PBL and feather cells but not in dust. VCI significantly influenced these relationships having strong positive and negative associations with load of vvMDV and Rispens respectively. Differences between the two viruses and their effects on each other were greatest in PBL and feathers, and least in dust. This study expands our understanding of the interaction between pathogenic and vaccinal viruses following vaccination with imperfect vaccines and has implications for selection of appropriate samples to test for vaccination success.